Thanks again Moo. My instrument has been built in 1999 by an italian amateur luthier and fine musician whose name is Giorgio Bionaz (I would be very surprised if you knew him). It comes with a fantastic hand made wooden case covered with a blue flowers tapestry; unfortunately the case is in very bad condition, otherwise I would be delighted to keep it. The instrument has not an adjustable trod.
About the string gauges, you suggested .052 .039 .025 and .013 to Inukshuk and me for GDAE tuning, and this I will try soon. I suppose that if I tune GDAD it should be the same, right? But when I will try GDGC heavier gauges would be suitable for the first two strings?
Thanks in advance for the trouble you take.